1. Field of the Invention:
The present invention relates generally to a display apparatus, and more particularly to a projection type display apparatus for displaying a projected color image through lenses from permissive display devices such as liquid crystal display devices wherein the color image is composed of multiple picture elements arranged in a matrix. This type of display apparatus is particularly applicable to a large-screen projection TV (television), information display systems and the like.
2. Description of the Prior Art:
The display devices used in the present invention do not emit light, but their transmissibility varies in response to a driving signal so as to display images and/or characters by modulating the intensity of an incident light. Examples of such display devices include liquid crystal displays, electrochromic displays, display units using transparent ceramics such as PLZT, among which the liquid crystal display devices are most widely used for pocket-sized TV and word processors. By taking the liquid crystal display panels as an example, the background of the present invention will be described:
The commonly called "matrix type liquid crystal display device" contains picture elements arranged in a matrix that are individually driven by driving voltages so as to change the optical characteristics of the liquid crystal so as to make up images and/or characters. The driving voltage can be applied to the individual picture elements by various means, for example, by a simple matrix system or alternatively, by an active matrix system under which a non-linear two-terminal element such as MIM (metal-insulating layer-metal) or three-terminal switching element such as TFT (thin film transistor) is disposed for each picture element. The present invention can be applied to either of these systems, but the greatest effect is achieved when it is applied to the active matrix system.
There are at least two methods by which the transmissive type display devices can be observed; one is to observe the display panel directly, and the other is to observe a projected image enlarged through a lens. The projection type is classified into two categories; one is to use a single liquid crystal display panel equipped with a mosaic color filter of primary colors (i.e. red, green and blue) and the other is to use three liquid crystal display panels each representing a monochrome image. In the latter projection system each display panel is illuminated with lights of primary colors, and makes up images of primary colors by representing a monochromatic image corresponding to color of the illumination. To compose the three images of primary colors, one method is to project the images individually onto a screen and compose them thereon, and the other method is to compose them by use of a dichroic mirror and project the composed image through a common projecting lens.
The above-mentioned single-panel system is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 49-74438 (allowed Publication No. 54-18886). This prior art discloses that thin film transistor controls picture panels individually, and an image is projected through color liquid crystal display devices equipped with filters of primary colors in mosaic and is observed. The employment of three liquid crystal display devices is disclosed in Japanese Laid-Open Patent Publication No. 60-179723.
The active matrix liquid crystal display (LCD) must employ signal lines among picture elements so as to drive a non-linear two-terminal element such as MIM (metal-insulating layer-metal) or three-terminal switching element such as TFT (thin film transistor) disposed for each picture element. The provision of signal lines requires spaces, but the allowed space is limited in view of the electric performance and manufacturing convenience. If the picture element pitches are excessively minimized, the aperture ratio (the ratio of an effective opening occupied by each picture element or an area for allowing the passage of light to the total area) is reduced. Part of incident rays to areas other than the picture elements is not subjected to modulation by display signals, and this part of the rays does not pass through the display element designed to display only in a "normally black mode" (a mode in which no ray is allowed to pass when no electric field is applied to the liquid crystal layer). When the part of the rays is passed through a display panel designed to display only in a "normally white mode" (a mode in which the rays are allowed to pass when no electric field is applied to the liquid crystal panel), the contrast of the image is reduced because of the increased black level. To avoid poor contrast of image, areas other than the picture elements are masked so as to absorb or reflect rays which do not contribute to the representation of images. In either case, the image is darkened because of the reduced aperture ratio throughout the display element. This defect is the same whether it is the directly observing type or the projection type.
As is the case with most conventional motion picture projectors and slide projectors, the projection type liquid crystal display apparatus use Kohler's illumination. To use this illumination, the optical system is constructed so that an image of the light source through a condenser lens is made up near projecting lens so as to minimize the diameter of the projection lens. If under this optical system a microlens array having a microlens for each picture element is used, the rays entered in the display devices are converged in each picture element area and modulated in response to a display signal, but after passing through the display areas, they diverge at angles depending upon numerical aperture (NA) of the microlens. When this angle is excessively large, a part of the rays passing through the display devices does not enter into the entrance pupil of the projecting lens, thereby resulting in dark images.
The numerical aperture (NA) is expressed by: ##EQU1## where R is the radius of the microlens and f is the focal length of the microlens, and F is the F number of microlens.
Japanese Laid-Open Patent Publication (Kokai) No. 60-262131 discloses a pair of microlens arrays disposed at each side of the display device. The first microlens array converges a light entering the display device at a picture element area thereof, and the second microlens array converts the diverging rays at an angle determined by the numeral aperture of microlens array. This display apparatus requires a precise alignment of the display element and a pair of microlens arrays. Furthermore, the light passing through the display device is not exactly converted into a parallel flux owing to a possible aberration of the microlens array. In addition, the increased number of interfaces is likely to cause reflection loss and scattering of light.